System and method for calibrating a spatial light modulator array using shearing interferometry

ABSTRACT

A system for calibrating a spatial light modulator array includes an illumination system and a spatial light modulator array that reflects or transmits light from the illumination system. A projection optical system images the spatial light modulator array onto an image plane. A shearing interferometer creates an interference pattern in the image plane. A controller controls modulation of elements of the spatial light modulator array. The shearing interferometer includes a diffraction grating, a prism, a folding mirror or any other arrangement for generating shear. The shearing interferometer can be a stretching shearing interferometer, a lateral shearing interferometer, or a rotational shearing interferometer. The shearing interferometer may include a diffraction grating with a pitch corresponding to a shear of the light by an integer number of elements. The projection optics resolves each element of the spatial light modulator array in the image plane. The controller can modulate alternate columns of elements of the spatial light modulator array.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to lithography. Moreparticularly, the present invention relates to wavefront aberrationmeasurement for purposes of SLM calibration in maskless lithography.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays (e.g., liquid crystal displays), circuit boards,various integrated circuits, and the like. A frequently used substratefor such applications is a semiconductor wafer or glass substrate. Whilethis description is written in terms of a semiconductor wafer forillustrative purposes, one skilled in the art would recognize that thisdescription also applies to other types of substrates known to thoseskilled in the art.

During lithography, a wafer, which is disposed on a wafer stage, isexposed to an image projected onto the surface of the wafer by exposureoptics located within a lithography apparatus. While exposure optics areused in the case of photolithography, a different type of exposureapparatus can be used depending on the particular application. Forexample, x-ray, ion, electron, or photon lithography each can require adifferent exposure apparatus, as is known to those skilled in the art.The particular example of photolithography is discussed here forillustrative purposes only.

The projected image produces changes in the characteristics of a layer,for example photoresist, deposited on the surface of the wafer. Thesechanges correspond to the features projected onto the wafer duringexposure. Subsequent to exposure, the layer can be etched to produce apatterned layer. The pattern corresponds to those features projectedonto the wafer during exposure. This patterned layer is then used toremove or further process exposed portions of underlying structurallayers within the wafer, such as conductive, semiconductive, orinsulative layers. This process is then repeated, together with othersteps, until the desired features have been formed on the surface, or invarious layers, of the wafer.

Step-and-scan technology works in conjunction with a projection opticssystem that has a narrow imaging slot. Rather than expose the entirewafer at one time, individual fields are scanned onto the wafer one at atime. This is accomplished by moving the wafer and reticlesimultaneously such that the imaging slot is moved across the fieldduring the scan. The wafer stage must then be asynchronously steppedbetween field exposures to allow multiple copies of the reticle patternto be exposed over the wafer surface. In this manner, the quality of theimage projected onto the wafer is maximized.

Conventional lithographic systems and methods form images on asemiconductor wafer. The system typically has a lithographic chamberthat is designed to contain an apparatus that performs the process ofimage formation on the semiconductor wafer. The chamber can be designedto have different gas mixtures and grades of vacuum depending on thewavelength of light being used. A reticle is positioned inside thechamber. A beam of light is passed from an illumination source (locatedoutside the system) through an optical system, an image outline on thereticle, and a second optical system before interacting with asemiconductor wafer.

A plurality of reticles are required to fabricate a device on thesubstrate. These reticles are becoming increasingly costly and timeconsuming to manufacture due to the feature sizes and the exactingtolerances required for small feature sizes. Also, a reticle can only beused for a certain period of time before being worn out. Further costsare routinely incurred if a reticle is not within a certain tolerance orwhen the reticle is damaged. Thus, the manufacture of wafers usingreticles is becoming increasingly, and possibly prohibitively expensive.

In order to overcome these drawbacks, maskless (e.g., direct write,digital, etc.) lithography systems have been developed. The masklesssystem replaces a reticle with a spatial light modulator (SLM) (e.g., adigital pixel device (DMD), a liquid crystal display (LCD), a gratinglight valve (GLV) or the like). The SLM includes an array of activeareas (e.g., tilting and/or pistoning mirrors or greytoning LCD arraycells) that vary optical properties in a controlled fashion to form adesired pattern.

Conventional SLM-based writing systems (e.g., Micronic's Signal 7000series tools) use one SLM as the pattern generator. To achieve linewidthand line placement specifications, gray scaling is used. For analogSLMs, gray scaling is achieved by controlling mirror tilt angle (e.g.,Micronic SLM) or polarization angle (e.g., LCD). For digital SLMs (e.g.,TI DMD), gray scaling is achieved by numerous passes or pulses, wherefor each pass or pulse the pixel (micromirror) can be switched either ONor OFF (for a binary SLM, or some in-between state for other SLMs)depending on the level of gray desired. Because of the total area on thesubstrate to be printed, the spacing between active areas, the timing oflight pulses, and the movement of the substrate, several passes of thesubstrate are required to expose all desired areas. This results in lowthroughput (number of pixels packed into an individual opticalfield/number of repeat passes required over the substrate) and increasedtime to fabricate devices. Furthermore, using only one SLM requires morepulses of light or more exposure time to increase gray scale. This canlead to unacceptably low levels of throughput.

Therefore, what is needed is a maskless lithography system and methodthat can expose all desired areas on a substrate for each pattern duringonly one pass of a substrate.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for calibratinga spatial light modulator array using shearing interferometry thatsubstantially obviates one or more of the problems and disadvantages ofthe related art.

The present invention includes a system for calibrating a spatial lightmodulator array with an illumination system and a spatial lightmodulator array that reflects or transmits light from the illuminationsystem. A projection optical system images the spatial light modulatorarray onto an image plane. A shearing interferometer creates aninterference pattern in the image plane. A controller controls tilting,pistoning, and/or deformation of elements of the spatial light modulatorarray. The shearing interferometer includes an arrangement forgenerating shear, e.g., a diffraction grating, prisms, folding mirrors,etc. The shearing interferometer can be, for example, a stretchingshearing interferometer, a lateral shearing interferometer, a radialshearing interferometer, or a rotational shearing interferometer. Theshearing interferometer may, but not necessarily, include a diffractiongrating with a pitch corresponding to a shear of the light by an integernumber of elements. The projection optics resolves each element of thespatial light modulator array in the image plane. The controller, in oneembodiment, modulates alternate columns of elements of the spatial lightmodulator array.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 shows a maskless lithography system having reflective spatiallight modulators.

FIG. 2 shows a maskless lithography system having transmissive spatiallight modulators.

FIG. 3 shows another illustration of a spatial light modulator accordingto an embodiment of the present invention.

FIG. 4 shows more details of the spatial light modulator of FIG. 3.

FIG. 5 shows a two-dimensional array of the spatial light modulatoraccording to one embodiment of the present invention.

FIG. 6 illustrates a portion of a reflective SLM of one embodiment ofthe present invention.

FIG. 7 illustrates a field in a pupil of the projection optics for tendifferent tilt values, for a large numerical aperture projection optics.

FIG. 8 illustrates a field in the projection optics image plane thatcorresponds to FIG. 7.

FIG. 9 illustrates a field in a pupil of the projection optics for tendifferent tilt values for a small numerical aperture projection optics.

FIG. 10 illustrates a field in the projection optics image plane thatcorresponds to FIG. 9.

FIGS. 11-12 illustrate an example of a shearing interferometerarrangement that may be used in the present invention.

FIG. 13 illustrates a grating that may be used in the shearinginterferometer of FIGS. 11-12.

FIG. 14 illustrates a field in a pupil of the projection optics for tendifferent tilt values in the system of the present invention.

FIG. 15 illustrates a field in the projection optics image plane thatcorresponds to FIG. 14.

FIG. 16 is an illustration of the image cross-section of one pixel inthe image plane that corresponds to FIG. 15.

FIG. 17 illustrates a relative power in the image plane for the varioustilt angles that corresponds to FIG. 15.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

While specific configurations and arrangements are discussed, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the pertinent art will recognize that other configurationsand arrangements can be used without departing from the spirit and scopeof the present invention. It will be apparent to a person skilled in thepertinent art that this invention can also be employed in a variety ofother applications.

FIG. 1 shows a maskless lithography system 100 according to anembodiment of the present invention. System 100 includes an illuminationstem 102 that transmits light to a reflective spatial light modulator(SLM) 104 (e.g., a digital micromirror device (DMD), a reflective liquidcrystal display (LCD), or the like) via a beam splitter 106 and SLMoptics 108. SLM 104 is used to pattern the light in place of a reticlein traditional lithography systems. Patterned light reflected from SLM104 is passed through beam splitter 106 and projection optics (PO) 110and written on an object 112 (e.g., a substrate, a semiconductor wafer,a glass substrate for a flat panel display, or the like).

It is to be appreciated that illumination optics can be housed withinillumination system 102, as is known in the relevant art. It is also tobe appreciated that SLM optics 108 and projection optics 110 can includeany combination of optical elements required to direct light ontodesired areas of SLM 104 and/or object 112, as is known in the relevantart.

In alternative embodiments, either one or both of illumination system102 and SLM 104 can be coupled to or have integral controllers 114 and116, respectively. Controller 114 can be used to adjust illuminationsource 102 based on feedback from system 100 or to perform calibration.Controller 116 can also be used for adjustment and/or calibration.Alternatively, controller 116 can be used for controlling active devices(e.g., pixels, mirrors, locations, etc.) 302 (see FIG. 3, discussedbelow) on SLM 104, to generate a pattern used to expose object 112.Controller 116 can either have integral storage or be coupled to astorage element (not shown) with predetermined information and/oralgorithms used to generate the pattern or patterns.

FIG. 2 shows a maskless lithography system 200 according to a furtherembodiment of the present invention. System 200 includes an illuminationsource 202 that transmits light through a SLM 204 (e.g., a transmissiveLCD, or the like) to pattern the light. The patterned light istransmitted through projection optics 210 to write the pattern on asurface of an object 212. In this embodiment, SLM 204 is a transmissiveSLM, such as a liquid crystal display, or the like. Similar to above,either one or both of illumination source 202 and SLM 204 can be coupledto or integral with controllers 214 and 216, respectively. Controllers214 and 216 can perform similar functions as controller 114 and 116described above, and as known in the art.

Example SLMs that can be used in systems 100 or 200 are manufactured byMicronic Laser Systems AB of Sweden and Fraunhofer Institute forCircuits and Systems of Germany. A grating light valve (GLV) SLM isanother example of an SLM where the present invention is applicable.

Merely for convenience, reference will be made only to system 100 below.However, all concepts discussed below can also apply to system 200, aswould be known to someone skilled in the relevant arts.

FIG. 3 shows details of an active area 300 of SLM 104. Active area 300includes an array of active devices 302 (represented by dotted patternsin the figure). Active devices 302 can be mirrors on a DMD or locationson a LCD. It is to be appreciated that active devices 302 can also bereferred to as pixels, as is known in the relevant art. By adjusting thephysical characteristics of active devices 302, they can be seen asbeing either ON or OFF (for a binary SLM) or a state in-between ON andOFF for other SLMs. Digital or analog input signals based on a desiredpattern are used to control various active devices 302. In someembodiments, an actual pattern being written to object 112 can bedetected and a determination can be made whether the pattern is outsidean acceptable tolerance. If so, controller 116 can be used to generateanalog or digital control signals in real time to fine-tune (e.g.,calibrate, adjust, etc.) the pattern being generated by SLM 104.

FIG. 4 shows further details of SLM 104. SLM 104 can include an inactivepackaging 400 surrounding active area 300. Also, in alternativeembodiments, a main controller 402 can be coupled to each SLM controller116 to monitor and control an array of SLMs, as discussed below. Alsodiscussed below, adjacent SLMs may be offset or staggered with respectto each other in other embodiments.

FIG. 5 shows an assembly 500 including a support device 502 thatreceives an array of SLMs 104. In various embodiments, as described inmore detail below, the array of SLMs 104 can have varying numbers ofcolumns, rows, SLMs per column, SLMs per row, etc., based on a number ofdesired exposures per pulse, or other criteria of a user. The SLMs 104can be coupled to a support device 502. Support device 502 can havethermal control areas 504 (e.g., water or air channels, etc.), areas forcontrol logic and related circuitry (e.g., see FIG. 4 showing elements116 and element 402, which can be ASICs, A/D converters, D/A converters,fiber optics for streaming data, etc.), and windows 506 (formed withinthe dashed shapes) that receive SLMs 104, as is known in the relevantart. Support device 502, SLMs 104, and all peripheral cooling or controldevices are referred to as an assembly. Assembly 500 can allow for adesired step size to produce the desired stitching (e.g., connecting ofadjacent elements of features on object 112) and overlap for leading andtrailing SLMs 104. By way of example, support device 502 can havedimensions of 250 mm×250 mm (12 in×12 in) or 300 mm×300 mm (10 in×10in). Support device 502 can be used for thermal management based onbeing manufactured from a temperature stable material.

Support device 502 can be utilized as a mechanical backbone to ensurespacing control of SLMs 104 and for embedding the circuitry and thethermal controls areas 504. Any electronics can be mounted on either orboth of a backside and front side of support device 502. For example,when using analog based SLMs or electronics, wires can be coupled fromcontrol or coupling systems 504 to active areas 300. Based on beingmounted on support device 502, these wires can be relatively shorter,which reduces attenuation of analog signals compared to a case where thecircuitry is remote from the support device 502. Also, having shortlinks between the circuitry and active areas 300 can increasecommunication speed, and thus increase pattern readjustment speed inreal time.

In some embodiments, when SLM 104 or electrical devices in the circuitrywear out, assembly 500 can easily be replaced. Although it would appearreplacing assembly 500 is more costly than just a chip on assembly 500,it is in fact easier and quicker to replace the entire assembly 500,which can save production costs. Also, assembly 500 can be refurbished,allowing for a reduction in replacement parts if end users are willingto use refurbished assemblies 500. Once assembly 500 is replaced, onlyverification of the overall alignment is needed before resumingfabrication. In some examples, kinematic mounting techniques can be usedto allow for repeatable mechanical alignments of assembly 500 duringfield replacements. This may eliminate a need for any optical adjustmentof assembly 500.

Current SLM systems typically utilize 16 μm×16 μm pixels 302 (see FIG.6), with next generation SLM systems moving to 8×8 μm pixels 302. Atypical SLM 104 contains millions of pixels 302, wherein the propertiesof each pixel 302 are individually controlled by a voltage appliedindividually to each pixel 302. Note that SLM 104 can be both reflectiveand transmissive (for example, mirror type reflective SLMs, and LCD typetransmissive SLMs). Reflective SLMs 104 are more commonly used in theindustry today. FIG. 6 is an illustration of such a reflective, ortilting, type SLM 104, showing twelve pixels (of which 302 a-302 d arelabeled). (Note that the invention is not limited to the pistoning typeSLMs, but is also applicable to tilting SLMs as well as SLMs withotherwise deformable mirrors.) A capacitive coupling (not shown) iscontrolled using a transistor (not shown). A typical pixel 302 iscontrolled in a fashion similar to how parallel plates in a capacitorare controlled, in other words, a capacitive coupling is used to controlthe tilt of the mirrors of the pixels 302 using electrostatic forces. InFIG. 6, one of the mirrors (mirror of pixel 302 d), is shown as tiltedwhen the capacitor under that mirror is charged.

A particular problem that exists with the use of SLMs 104 is one ofcalibration of the individual elements or pixels 302. In the case of apiston type, or tilting mirror type, SLMs, the exact dependence of eachmirror of each pixel 302 on the applied voltage may vary. Additionally,the mirrors are not entirely “solid” objects, and deform somewhat (inaddition to tilt) when a voltage is applied. Furthermore, there are gapsbetween adjacent pixels 302, which degrade the optical performance dueto additional reflections from “in between” the pixels 302.

Although the principal effect of changing the voltage applied to thepixel 302 is in the tilting affect, the deformation of the mirror of thepixel 302 is a secondary effect, but may be a substantial enough effectso as to affect the quality of the image. Thus, the problem stated inits most general form is one of determining the dependence of theorientation and shape of the mirror on the applied voltage, for eachmirror in the SLM 104.

Each mirror changes the shape of its reflecting surface depending on thevoltage (V_(j)) applied to its pixel 302: $\begin{matrix}{{S_{j}\left( {x,y,V_{j}} \right)} = {\underset{\underset{{{principal}\quad{dependence}},{{same}\quad{for}\quad{all}\quad{mirror}\quad{pixels}}}{︸}}{S^{(0)}\left( {{x - x_{j}},{y - y_{j}},V_{j}} \right)} + \underset{\underset{{{individual}\quad{variations}\quad{of}}{{the}\quad{pixels}^{\prime}\quad{properties}}}{︸}}{S_{j}^{({ind})}\left( {{x - x_{j}},{y - y_{j}},V_{j}} \right)}}} \\{= {\underset{\underset{{{tilt},\quad{{the}\quad{principal}}}{effect}}{︸}}{{T_{j}\left( V_{j} \right)} \cdot \left( {x - x_{j}} \right)} + \underset{\underset{{resisual}\quad{shape}\quad{variation}}{︸}}{R_{j}\left( {{x - x_{j}},{y - y_{j}},V_{j}} \right)}}} \\{{{T_{j}\left( V_{j} \right)} = {{T^{(0)}\left( V_{j} \right)} + {T_{j}^{({ind})}\left( V_{j} \right)}}},}\end{matrix}$

Diffraction field from a single mirror in the PO 110 entrance pupil isgiven by: $\begin{matrix}{{U_{j}\left( {f_{x},f_{y},V_{j}} \right)} = {{FT}\left( {\exp\left( {{{\mathbb{i}} \cdot 2}{\pi \cdot {{S_{j}\left( {x,y,V_{j}} \right)}/\lambda}}} \right)} \right)}} \\{= {\underset{\underset{{{{principal}\quad{mechanical}\quad{effect}},{{the}\quad{tilt}},\quad{{results}\quad{in}\quad a\quad{shift}\quad{of}\quad{the}}}{{field}\quad{pattern}\quad{in}\quad{the}\quad{pupil}}}{︸}}{U^{(0)}\left( {{f_{x} + {{T_{j}\left( V_{j} \right)}/\lambda}},f_{y}} \right)} + \underset{\underset{{{residual}\quad{shape}\quad{variations}}{{result}\quad{in}\quad a\quad{deformation}\quad{of}}{{the}\quad{diffraction}\quad{pattern}}}{︸}}{U_{j}^{({ind})}\left( {f_{x},f_{y},V_{j}} \right)}}}\end{matrix}$where (f_(x) ²+f_(y) ²)^(1/2)<(1+σ)·NA/λ, and a corresponds to typicalPO 110 illumination. Note that for purposes of calibration, the PO 110used normally in the tool may be replaced (or augmented with) auxiliarycalibration optics. The auxiliary calibration optics in the ML Tool aretypically needed to resolve pixels 302, since the regular projectionoptics of the tool usually cannot resolve pixels 302, and because ashearing interferometer might need more space than is availableotherwise.

Only the variation of the diffraction field within the PO 110 entrancepupil is of significant importance. Thus, high-frequency variations ofthe reflective surface shape of the mirror are less important than theprincipal effect (the mirror tilt). In practical terms, the mirrors areoperated not as “analog” devices, but more akin to digital devices,with, for example, each mirror oriented in one of 64 possibleorientations. For a 1,000×1,000 mirror SLM 104, this results in needingto know the dependence of the SLM 104 on 64,000,000 voltages. Analternative way to formulate this problem is to require that the angularorientation of each mirror for some specific voltage is the same for allmirrors in the SLM 104. However, this may require applying differentvoltages to different pixels 302, because different pixels 302 may havea different response to the same voltage, due to a variation in themechanical properties from one mirror to another.

Thus, knowledge of the shape assumed by each mirror in the SLM 104 inresponse to the voltages applied can be used for calibration of eachmirror to result in the same tilt response for each pixel 302 in the SLM104.

Also, the process of calibration can identify any pixels 302 that arebroken and/or not functioning properly. Furthermore, even when novoltage is applied to any of the pixels 302, the surface of the SLM 104is not a perfectly flat mirror, but rather has height variations acrossthe surface, which also potentially degrade image quality. This isprimarily due to the imperfections in the pixels and deficiencies in themanufacturing process. This is another issue that should be addressed bythe calibration process.

For all current practical purposes, it is therefore sufficient to solvethe following problem:

For each pixel 302, that is pixel_(j), where j=1, . . . , N, determinethe set of voltage levels, V_(j) ^((n)), where n=1, . . . , N, such thatany two different pixels j1 and j2 with the voltages of the same levelsV_(j1) ^((n)) and V_(j2) ^((n)) applied to them will produce (almost)the same image fields.

A more general problem, the solution of which would fully describe theimaging properties of the SLM 104, is as follows: for each pixel 302,find the dependence of its image field on voltage (within a certainrange of voltage variation), applied to this pixel.

The shape of each mirror is measured by analyzing the image created bythat mirror in the image plane. Another way to view the calibrationproblem is as follows: if there are (e.g.) 64 voltages, such that eachvoltage corresponds to a certain mirror position, we need to determinethe 64 voltages for each mirror such that all the mirrors have theidentical position (or orientation) for each voltage. In order toaccomplish this goal, that is, in order to orient the mirrors in thesame direction (at some voltage), it is necessary to know thecharacteristics of the mirrors, such as mirror response to the voltagesand the deformation of each individual mirror at the particularvoltages.

It is possible, in theory, to turn off all the pixels 302 except for theone pixel 302 being calibrated, and by measuring the image of that onepixel 302 in the image plane at various voltages, calibrate that pixel302 very precisely. However, given the fact that the pixels 302 numberin the millions, doing this process pixel-by-pixel can be time consumingor even impractical. A somewhat intermediate approach would be tocalibrate several pixels 302 at a time, where the pixels 302 beingcalibrated are separated far enough from each other such that they donot interfere with each other in the image plane. However, this processcan still be fairly time-consuming.

The second problem of calibration is particularly relevant to reflectivetilting micromirror type SLMs, and has to do with how the pixels 302 areimaged. It is important that the pixels 302 be imaged in such a way thatthey are not resolved by the projection optics 110. In order to modulatethe light, the pixel 302 should not be resolved. If all the light fromthe tilting pixel 302 d is captured by the projection optics 110, thepixel 302 d will not modulate. If the pixel 302 is a square, itsdiffraction pattern is a sinc function, with a large zeroth order lobe,and smaller side lobes. When a pixel 302 is tilted, the diffractionpattern from the pixel 302 shifts an angular space to the side. With alarge enough entrance pupil of the projection optics 110, many sidelobes from the diffraction pattern will be captured (if the pixel 302 dis tilted at a relatively small angle only). Thus, in this case, theamount of light captured by the projection optics 110 does not vary, andthe pixel 302 will not modulate. For practical purposes, the pixel 302has to be imaged in such a way that it is not resolved.

On the other hand if the projection optics 110 only captures a portionof the zeroth order lobe, for example, ½ or ⅓ of the total amount ofenergy in the zeroth order lobe, then tilting the pixel 302 d modulatesthe amount of light passing through the projection optics 110. Thus, itis essential for the modulation mechanism that the pixel 302 d not beresolved, in order to have a modulation effect. However, because thepixel 302 d is not resolved, instead of seeing a “sharp square” (for asquare pixel or mirror), a “blob” of light will be imaged, and willexceed the nominal dimensions of the “sharp square” by several times.Thus, images from neighboring pixels 302 will overlap. The neighboringpixels 302 therefore will strongly interact with each other. Thiscreates a problem with modulation, because at each point in the imageplane, light is received from several pixels 302.

These two considerations make the problem of calibration difficultresolving the pixel 302 into a “sharp square” means that there is noeffective modulation, and effective modulation means that the pixel 302cannot be resolved. Phrased another way, either one has a resolutionthat one desires, or sensitivity to a shape of the pixel, but not both.

For the example illustrated in FIGS. 6-8, λ=193.375 nm, L (pixeldimension)=16 μm, NA (calibration PO)=10*λ/L=0.12 (i.e., the calibrationPO 110 captures up to the 10th diffraction order, which means itresolves the pixel 302 well), pixel 302 tilts between a=0 and a=a₀=λ/(2L)—the range of tilts to calibrate. FIG. 7 illustrates the field in thepupil of the projection optics 110 for ten different tilt values for asingle pixel (note that with this and subsequent related figures,modulation of only one pixel is illustrated for clarity, although theinvention permits measurement of multiple pixels simultaneously). With anumerical aperture of 0.12 (a fairly large numerical aperture), thepixel 302 is well defined in the PO pupil field for all the tilt anglesillustrated. However, as shown in FIG. 8, in the PO image plane, thereis virtually no modulation of intensity for the entire angular range. Inother words, with so many diffraction orders captured by the largenumerical aperture projection optics 110, modulation is not achieved,even though the pixel 302 is well resolved in the PO image plane.

FIG. 9 illustrates the “opposite case” of using a very small numericalaperture, in this case a numerical aperture of 0.00265. Here, the fieldis resolved in the pupil plane (see FIG. 9) but is very poorly resolvedin the image plane (see FIG. 10) for ten different tilt angles.Specifically, as shown in FIG. 10, although there is good modulation forthe different tilt angles, the image of the pixel 302 is very “spreadout,” which means that adjacent pixels 302 would interfere with eachother if imaged simultaneously, and that measuring the distortion andshape of the reflections across each pixel 302 would be difficult.

The solution to this problem is the utilization of planarinterferometry. If the SLM 104 is visualized as a perfect mirror, and aplanar wave is directed at the SLM 104, the reflected wave should alsobe planar. If one or more of the mirrors is tilted, or pistoned, thereflected wave will be formed accordingly. In other words, a mirroradjustment corresponds to having an aberration in the reflected planarwavefront. Thus, conceptually, modulation of the SLM 104 can be viewedas the introduction of an aberration into the wavefront. Measuring themodulation effect of the SLM 104 can be viewed as the measurement ofwavefront aberration using interferometry. In this case, unliketraditional wavefront aberration measurements, the spatial frequency iscomparable to the dimension of the mirror, i.e., the dimension of thepixels 302.

A simple type of an interferometer that can be used to measure wavefrontaberrations is a Michelson interferometer, where a wavefront isseparated (for example, using a beam splitter), then one half of thewavefront passes through a medium that introduces aberrations, and isthen combined with the unaberrated half of the wavefront. Thus, phasedistortion becomes apparent in terms of intensity variation in the imageplane. A Michelson interferometer, which uses interference between areference wavefront and a reflected wavefront (from the SLM 104) can, inprinciple, be used to calibrate the SLM 104. Using the Michelsoninterferometer approach, it is possible to determine the deviation fromperfect flatness (planarity) of the SLM 104 by combining a referencewavefront with a wavefront reflected off the SLM 104 when no voltage isapplied. After determining the deviation from perfect planarity of theSLM 104, the rest of the measurements can be performed by combiningwavefronts obtained with tilted mirrors with a reference wavefront.

The use of a Michelson interferometer is not without its difficulties.It requires additional optical elements, such as beam splitters, etc. Italso requires addressing the issue of temporal coherence in the incomingwavefront. The advantage of shearing interferometry is that instead ofinterfering a referenced wavefront with the reflected wavefront, whichrequires the addition of optical elements, in the shearinginterferometry approach, the wavefront interferes with a sheared copy ofitself

A shearing interferometer can use a grating to shift the wavefrontlaterally. Other arrangements are possible, for example, rotation of thewavefront, radial displacement of the wavefront, stretching of thewavefront, “flipping” of the wavefront, etc. By interfering the shearedwavefront with the original wavefront, it is possible to produceinterference fringes in the image plane, which enable one to determineaberrations in the wavefront, and in turn enable one to determine thedependence of the mirror tilt on the applied voltage. The simplestarrangement is the use of a shearing grating placed in the pupil of theprojection optics 110. This produces two wavefronts, which are laterallyshifted with respect to each other. Thus, even if the pixels 302 areresolved in the image plane, it is possible to measure the dependence ofthe tilt of each mirror on the applied voltage.

FIGS. 11 and 12 illustrate the use of a pupil in a lateral shearinginterferometer 1110 to produce reference wavefronts and shearwavefronts. As shown in FIGS. 11 and 12, a wavefront 1101 converges at apoint in space, while emanating from a primary source. An image of apoint source exists at an entrance pupil. A partially transmitting filmmay be placed at the entrance pupil. A pinhole 1103 is positioned at theentrance pupil. The pinhole 1103 generates a transmitted wave 1104 witha wavefront 1111, which includes a diffracted spherical reference wave1105. Thus, the lateral shearing interferometer 1110 creates one or moreapparent sources, whose wavefronts interfere to produce fringes.

An image shearing mechanism 1201 (see, for example, the gratingillustrated in FIG. 13) is positioned within the shearing interferometer1110, and generates the multiple wavefronts 1104, 1105 that are thendetected by a CCD detector in the image plane (not shown). The imageshearing mechanism 1201 may be a diffraction grating, a prism, a foldingmirror, or any other device used for generating shear. Note that for SLMcalibration, it is necessary to measure the aberrations occurring in theobject plane of the PO 110. Therefore, the shearing grating should beplaced in the pupil of the PO 110 and the interferograms will beobserved in the image plane.

As explained above, the modulated SLM 104 results in an introduction ofa certain wavefront aberration that depends on the modulated states ofits pixels 302. In order to calibrate the SLM 104, the actual modulatedstates of each pixel 302 resulting from a certain driving voltagesapplied to them, need to be determined. The shearing interferogramsmeasured in the image plane can be interpreted, using any number ofknown methods, to derive the wavefront aberration from the measuredshearing interferograms. The result of this interpretation will yieldthe actual modulated state of each pixel 302.

Phase stepping technique used in lateral shearing interferometry can beused to improve the accuracy and to separate the interference fromhigher diffraction orders (if a shearing grating in the pupil is used toprovide a shear). Phase stepping can be implemented by either shiftingthe pupil apodization grating in small steps within one grating period,or by shifting the object (SLM array 104), or by tilting theillumination.

A partially coherent illumination of the calibrated SLM 104 may beneeded to reduce the effect of the flare in the PO 110 or a speckle,observed with coherent illumination. If a shearing grating in the pupilof the PO 110 is used to provide shear, a partially coherentillumination source will reduce the contrast of the interferogram. Thiscan be countered by providing partially coherent illumination from anextended source that is modulated by a grating with a period thatmatches the period of the shearing grating in the pupil (similar to theuse of matching pair of Ronchi gratings in lateral shearinginterferometry).

One embodiment of the present invention can use an arrangement where allpixels 302 of the SLM 104 are calibrated simultaneously, or where onlyeven columns of pixels 302 are tilted, while odd columns are not (or,equivalently, odd and even rows). In the image plane, modulated pixels302 will overlap with the unmodulated ones. The images in the imageplane therefore are the result of an interference between a modulatedpixel 302 and a neighboring unmodulated one. This is somewhat analogousto the Michelson interferometer arrangement, where an aberratedwavefront interferes with a planar wavefront (in this case, coming fromthe untilted mirror). This permits a determination of the shape of themodulated pixel 302 d for each voltage, which corresponds to aparticular tilt angle and mirror shape. In other words, the shear iseffected by an integer number of pixels 302.

Another embodiment utilizes shearing by a fraction of a pixel 302 (e.g.,¼, or ⅓), such that each pixel 302 interferes primarily with itself (theamount of the shear depends on the pitch of the grating 1201 of FIG.13).

The advantage of the approach described above is in enablingsimultaneous measurement of a very large number of pixels 302(potentially all of the pixels 302 of the SLM 104). Also, the aboveapproach solves the problem of being unable to resolve the pixels 302sufficiently in the image plane. The variation of intensity within each“square” corresponds to the shape of each pixel 302.

Because the pixels 302 are resolved, there is no need to iterate or usea large number of non-interfering groups of pixels 302. Also, each pixel302 is calibrated individually, without basing a calibration procedureon a specific pattern. The SLM however needs to be imaged in a way thatis different from the way it is imaged in the tool. This can be achievedby introducing a special calibration PO section between the SLM and thetool PO 110 (during the calibration procedure only).

Because the pixels 302 are resolved during the calibration, their imagesdepend on the shape and magnitude of the higher diffraction orders. Aspecial consideration needs to be given to account for the dependence ofthe measured resolved image on the higher diffraction orders (which arenot captured by the tool PO).

The effect of adding the shearing interferometer 1110 with the grating1201 that shears the beam laterally by ¼ of the pixel 302 width, isillustrated in FIGS. 14 and 15. FIG. 14 illustrates the field in thepupil plane for ten different tilt values. Note that some of the imagesare “partial,” due to the one-quarter pitch diffraction grading, asnoted above. In the image plane, as illustrated in FIG. 15, the pixel302 is well resolved (a square or rectangular shape is clearly visible),and at the same time there is good modulation for the different tiltangles, notwithstanding the relatively small numerical aperture. Inother words, the use of a shearing interferometer permits theachievement of both goals: good resolution in the image plane, combinedwith good modulation in the angular space.

FIG. 16 is an illustration of the intensity cross-section of one pixel302 in the image plane for the different tilt angles of the mirror. Thegraphs are inverted relative to the legend on the left-hand side of thefigure; in other words, the 0.0 tilt is at the bottom, and the 1.0relative tilt is at the top. FIG. 17 illustrates the relative power inthe image plane for the various tilt angles. Graph A illustrates thetotal power received within a specified area defined by pixel 302dimensions in the image plane, and graph B specifies the power outsidethat area. Graph C is the sum of graphs A and B. In the ideal case,graph B would be nearly perfectly flat and relatively low in magnitude,which is achieved here.

Although the discussion above is primarily in terms of calibrating atilting micromirrors-based SLM, the invention is also applicable toother types of SLMs, such as SLMs utilizing pistoning or otherwisedeformable micromirrors, or SLMs utilizing transmissive (refractive)pixels based on other modulation principles.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A system for calibrating a spatial light modulator array comprising:an illumination system; a spatial light modulator array that modulateslight from the illumination system; a projection optical system thatimages the spatial light modulator array onto an image plane; a shearinginterferometer that creates an interference pattern in the image plane;and a controller modulating elements of the spatial light modulatorarray.
 2. The system of claim 1, wherein the shearing interferometerincludes any of a diffraction grating, a prism, and a folding mirror forgenerating shear.
 3. The system of claim 1, wherein the shearinginterferometer is a lateral shearing interferometer.
 4. The system ofclaim 1, wherein the shearing interferometer is a stretching shearinginterferometer.
 5. The system of claim 1, wherein the shearinginterferometer is a rotational shearing interferometer.
 6. The system ofclaim 1, wherein the projection optics resolves each element of thespatial light modulator array in the image plane.
 7. The system of claim1, wherein the controller modulates alternate columns of elements of thespatial light modulator array.
 8. The system of claim 1, wherein theshearing interferometer includes a diffraction grating with a pitchcorresponding to a shear of the light by an integer number of elementsof the spatial light modulator array.
 9. The system of claim 1, whereinthe shearing interferometer includes a diffraction grating with a pitchcorresponding to a shear of the light by a fractional number of elementsof the spatial light modulator array.
 10. The system of claim 1, whereinthe controller controls any of tilting, pistoning, and deformation ofthe elements.
 11. The system of claim 1, further including a CCDdetector in the image plane for detecting the interferogram.
 12. Thesystem of claim 11, wherein the CCD detector includes a plurality ofdetector cells corresponding to each element of the spatial lightmodulator array.
 13. The system of claim 1, wherein the spatial lightmodulator array is a reflective spatial light modulator array.
 14. Thesystem of claim 1, wherein the spatial light modulator array is apistoning-type spatial light modulator array.
 15. The system of claim 1,wherein the spatial light modulator array is a tilting micromirror-typespatial light modulator array.
 16. The system of claim 1, wherein thespatial light modulator array is a transmissive spatial light modulatorarray.
 17. A method for calibrating a spatial light modulator arraycomprising: generating light from an illumination system; modulating thelight using a spatial light modulator array; passing the light through aprojection optical system so as to image the spatial light modulatorarray onto an image plane; shearing the light so as to create aninterference pattern in the image plane; detecting the light in theimage plane so as to measure interference fringes; and modulating thespatial light modulator array while repeating the detecting step. 18.The method of claim 17, wherein the shearing step includes shearing thelight using any of a diffraction grating, a prism and a folding mirror.19. The method of claim 17, wherein the shearing step includes shearingthe light using a stretching shearing interferometer.
 20. The method ofclaim 17, wherein the shearing step includes shearing the light using arotational shearing interferometer.
 21. The method of claim 17, applyingthe projection optics to resolve each element of the spatial lightmodulator array in the image plane.
 22. The system of claim 17, whereinthe shearing step includes shearing the light using a diffractiongrating with a pitch corresponding to a shear of the light by an integernumber of spatial light modulator array elements.
 23. The system ofclaim 17, wherein the shearing step includes shearing the light using adiffraction grating with a pitch corresponding to a shear of the lightby an fractional number of spatial light modulator array elements.